A whole lot of warmth packed into 3.4 oz (Size XS). Theresa just bought one of these recently and she absolutely loves it. It is implausibly light and kept her toasty warm as the temperatures dropped. I think this will quite possibly be her most worn article of clothing, and she may be able to eliminate her other fleece sweater over time.
It's made of 850+ Fill Power Goose Down, has reflective edging and an ultra-light shell.
Available from Backcountry Gear for $125.